cgroups: simplify init_subsys()

We are at system boot and there is only 1 cgroup group (i,e, init_css_set), so
we don't need to run through the css_set linked list.  Neither do we need to
run through the task list, since no processes have been created yet.

Also referring to a comment in cgroup.h:

struct css_set
{
	...
	/*
	 * Set of subsystem states, one for each subsystem. This array
	 * is immutable after creation apart from the init_css_set
	 * during subsystem registration (at boot time).
	 */
	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
}

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 10 insertions, 28 deletions

diff --git a/Documentation/cgroups.txt b/Documentation/cgroups.txt
index 31d12e21ff8a..c298a6690e0d 100644
--- a/Documentation/cgroups.txt
+++ b/Documentation/cgroups.txt
@@ -500,8 +500,7 @@ post-attachment activity that requires memory allocations or blocking.
 
 void fork(struct cgroup_subsy *ss, struct task_struct *task)
 
-Called when a task is forked into a cgroup. Also called during
-registration for all existing tasks.
+Called when a task is forked into a cgroup.
 
 void exit(struct cgroup_subsys *ss, struct task_struct *task)
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c447c29f8749..b893c8c94858 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2477,7 +2477,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
 {
         struct cgroup_subsys_state *css;
-        struct list_head *l;
 
         printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);
 
@@ -2488,35 +2487,19 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         BUG_ON(IS_ERR(css));
         init_cgroup_css(css, ss, dummytop);
 
-        /* Update all cgroup groups to contain a subsys
+        /* Update the init_css_set to contain a subsys
          * pointer to this state - since the subsystem is
-         * newly registered, all tasks and hence all cgroup
-         * groups are in the subsystem's top cgroup. */
-        write_lock(&css_set_lock);
-        l = &init_css_set.list;
-        do {
-                struct css_set *cg =
-                        list_entry(l, struct css_set, list);
-                cg->subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
-                l = l->next;
-        } while (l != &init_css_set.list);
-        write_unlock(&css_set_lock);
-
-        /* If this subsystem requested that it be notified with fork
-         * events, we should send it one now for every process in the
-         * system */
-        if (ss->fork) {
-                struct task_struct *g, *p;
-
-                read_lock(&tasklist_lock);
-                do_each_thread(g, p) {
-                        ss->fork(ss, p);
-                } while_each_thread(g, p);
-                read_unlock(&tasklist_lock);
-        }
+         * newly registered, all tasks and hence the
+         * init_css_set is in the subsystem's top cgroup. */
+        init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
 
         need_forkexit_callback |= ss->fork || ss->exit;
 
+        /* At system boot, before all subsystems have been
+         * registered, no tasks have been forked, so we don't
+         * need to invoke fork callbacks here. */
+        BUG_ON(!list_empty(&init_task.tasks));
+
         ss->active = 1;
 }